A random access transmission is used e.g. to set up a connection between a user equipment UE and a node providing a physical link between the user equipment and a communications network. In UMTS (Universal Mobile Telecommunications System) this node is called Node B. Analogies can be drawn between the function of a Node B and those of a Base Transceiver Station.
In general, an RACH (Random Access CHannel) carries uplink control information such as a request to set up an RRC (Radio Resource Control) connection. It is further used to send small amount of uplink packet data. Because of the relative large delay related to RLC (Radio Link Control)-level re-transmission, the BLER (BLock Error Rate) operation point of the RACH transmission must be kept at relative low level (<5%), otherwise the delays in connection setup will be rather high. FIG. 1 shows an RLC delay of 150 ms between a first transmission 11 of an RACH message 10 and a first re-transmission 12 of the same. Thus, a considerable delay between the time when the RACH message 10 arrives at a data buffer of the UE and a time when the RACH message 10 is correctly received by the Node B is caused when the RACH message has to be re-transmitted.
For avoiding re-transmission, rather high Eb/No (Energy per bit-to-Noise density ratio) requirement for the RACH reception is required. RACH performance issues are further emphasized by the fact that soft handoff and power control are not supported by the RACH.
Currently supported transport formats for RACH include TTI (Transmission Time Interval) lengths of 10 ms and 20 ms. Link level simulations were conducted to study the delay performance of RACH using 10 ms TTI with a data rate on RACH of 16 kbps. FIG. 2 illustrates the simulated function of average RACH delay and DCH (Dedicated CHannel) capacity loss in terms of 64 kbps users. The reference case corresponds the situation where no RACH load is present, giving a cell throughput value of 790 kbps for dedicated channels. Further simulation parameters and assumptions are 10% BLER, Noise rise=3 dB, ratio of other-to-own cell interference i=0.5, ITU Vehicular A channel, v=30 km/h, 2 Rx antennas, RLC delay=150 ms and average RACH arrival rate=1/2 times per frame.
As can be seen from FIG. 2, the higher the delay requirement for RACH, the higher is the capacity degradation experienced by dedicated channels. In other words, the L1 or physical layer delay of RACH can be improved at the expense of DCH capacity. FIG. 2 shows also the fact that the cell capacity is degraded when the RACH activity (Poisson rate) is increased (and vice versa). In other words, the data rate (16 kbps) used in RACH also has an impact on the DCH capacity.
FIG. 2 shows that the average RACH delay using the current transport format is always more than 20 ms. As illustrated in FIG. 1, delay components included in the average RACH delay are preamble ramping process 13, transmission/reception of the message part (>10 ms) and additional delay due to the RLC-level re-transmissions (150 ms each). It is to be noted that the numerical values of the delay components presented in FIG. 1 are examples. Actual values may depend on implementation.
A prior art technique to reduce the RACH delay is to use the shorter TTI. However, use of shorter TTI length means decreased coverage area for RACH and at the same time degraded radio performance because of the smaller interleaving gain. RACH coverage is essential for proper system operation because RACH must be heard from the whole desired cell coverage area.
Another prior art technique to reduce the RACH delay is to use L1 HARQ (Hybrid Automatic Repeat Request). This approach is more complex from the UE point of view since it requires data buffering and fast L1 feedback signaling between Node B and the UE.